google apps script - importXML is making too many Mapquest transactions -

i trying use following formula , key obtained mapquest (for free service) initiate location services , calculate distances between 1 variable location , 20 of plant locations.

=importxml("http://mapquestapi.com/directions/v2/route?key=*****************&outformat=xml&from=" & $b$3 & "&to=" & 56244,"//response/route/distance") 

this working flawlessly expect after using short period of time have received email stating have used 80% of allotment (15000 transactions) month.

the variable location has been changed around 20-25 times month don't see how have used many transactions. can explain formula doing , how make more efficient if possible? feel has using transaction unnecessary. keep in mind not need actual directions need driving mileage required.

thanks in advance.